The present invention relates to hearth rollers for conveying metal strips, and more particularly to hearth rollers having a suppressed heat crown in an area where a metal strip contacts the roller so that the roller can convey metal strips at a high temperature in a stable manner.
In processing lines such as those for continuous heat treating furnaces (e.g., continuous annealing furnaces) hearth rollers have been employed to convey metal strips (sometimes referred to as "strip" hereunder). In such conventional hearth rollers, a predetermined initial crown is provided in order to convey strips in a stable manner.
In a continuous annealing furnace, for example, hearth rollers provided within the furnace must carry metal strips having a variety of temperatures, widths, and thicknesses, and a heat crown is inevitably generated so that the initial roll crown of the roller cannot be maintained.
There are many problems caused by variation in roll crown. When the roll crown is excessive, buckling of strips referred to as "heat buckling" or "cooling buckling") occurs, resulting in troubles in conveying metal strips as well as degradation in the quality of the product strips. When the roll crown is too small, the centering effect is weakened, resulting in meandering of strips on the rollers.
One solution to such problems caused by the occurrence of heat crown is, as is disclosed in Japanese Patent Application Unexamined Specification No.61-210129/1986, to employ hearth rollers having a crown which can adjusted mechanically. As shown in FIG. 13, a tapered surface 10 formed on an inner portion of the hearth roller 9 engages with an adjusting member 12. The position of the adjusting member 12 is movable along the longitudinal axis of the roller by an adjusting means 14 provided outside the roller. The roll crown can be adjusted by controlling the axial position of the adjusting member 12 so that a sleeve 16 can be deformed. A variation of the roll crown is caused by the occurrence of a heat crown, which is caused by the occurrence of a variation in temperatures of the strip in the widthwise and longitudinal directions. In this hearth roller, such roll crown variations can be mechanically offset by the deformation of the sleeve caused by the movement of the tapered surface, and a given level of crown can be maintained.
Japanese Patent Application Unexamined Specification No. 63-65016/1988 discloses another hearth roller which can prevent a heat crown by obtaining a uniform thermal distribution throughout the roller. Such hearth rollers, as shown in FIG. 14, contain a molten metal 20 as a thermal medium within the roller 22 so that the temperature deviation in the axial direction of the roller barrel 24 can be diminished to suppress the occurrence of heat crown.
However, such hearth rollers as described above cannot prevent occurrence of heat crown effectively.
Namely, the hearth roller shown in FIG. 13 requires a control system which can estimate or measure a change in heat crown whenever it occurs so as to maintain a predetermined roll crown. Such a control system adds to costs. There are still other problems with respect to lubrication, endurance, and the like of a variable crown mechanism when a hearth roller is used in a high temperature atmosphere, such as in a continuous annealing furnace.
On the other hand, a hearth roller shown in FIG. 14 essentially utilizes the thermal content of the thermal medium. Since the thermal content is equal to the product of the specific heat and the mass, it is more advantageous to use a molten metal than a molten salt in order to make the temperatures of the roller uniform in the widthwise direction. This is because metals have a larger mass, i.e., a higher density than molten salts. However, when a metal is used, the mass distribution of the roller is not uniform, and centrifugal force acting on the molten metal causes vibration of the roller. In order to prevent the roller from vibrating it is necessary to employ additional equipment such as bearings, which adds to costs. In the case of a molten salt, its effectiveness at producing uniformity of the roller temperature is degraded to some extent but costs are reduced compared with when a molten metal is used.